Method of operating high pressure steam generators



United States Patent Ofiice 3,356,073 METHOD OF OPERATING HIGH PRESSURE STEAM GENERATORS Mortimer C. Bloom, Washington, D.C., assignor to the United States of America as represented by the Secretary of the Navy No Drawing. Filed June 30, 1966, Ser. No. 562,950 3 Claims. (Cl. 122379) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to the operation of steam generators, more particularly to the provision of corrosion protection to steel heating surfaces during high pressure operation.

General practice in the operation of steam boilers for power purposes is to use boiler water which is alkalized with sodium hydroxide, for protection against acid corrosion. The boiler water generally also contains a sodium phosphate for producing a soft form of boiler scale from calcium and magnesium hard water salts which is removed on blow down of the boiler.

Steam boilers have been operated at increasingly higher pressures to meet steadily rising power demands. Present day boiler operating pressures are in the range of 1200 to 1400 psi. and higher. Operation of steam boilers at the higher pressures introduced a type of boiler corrosion, known as pitting corrosion, in which locally developed strong solutions of caustic soda attack the steel and deeply pit the tube walls at irregularly occurring areas.

It has been recently found that steam boilers can be operated at high pressures without incurring pitting corrosion of the steel tubes by using feed water which is alkalized with lithium hydroxide instead of with sodium hydroxide. In this method of operating steam boilers at high pressures, described in U. S. Patent 3,173,404 to Bloom, Krulfeld, and Newport, granted March 16, 1965, protection against pitting corrosion is provided by the deposition on the surface of the heated steel tubes during boiler operation of an adherent tight film of a lithium enriched spinel which has the same structure but less lithium than LiFe O The lithium enriched spinel is formed by incorporation of lithium into Fe O spinel from the initial corrosion of the steel tubes by the alkalized boiler water. Formation of the lithium enriched spinel occurs only at sites on the tube surfaces where a trong solution containing from about 0.5 to 1% L OH is developed by local overheating of the tubes.

It has since been found that operation of steam boilers at high pressures using feed water alkalizcd with lithium hydroxide and containing a sodium phosphate in conventional low concentrations as used for forming a soft boiler scale from calcium and magnesium hard Water salts will produce extensive deposits of lithium phosphate which form a hard scale on heat transfer surfaces. Such deposits would cause tube failures due to overheating of the metal.

It is an object of the present invention to provide an improved method of operating steam generators at high pressures. It is also an object to provide a method of this kind in which the advantages of the use of lithium hydroxide as the alkalizer for the feed water are retained along with the prevention of boiler scale due to calcium and magnesium hard water salts.

I have found that steam generators can be operated at high pressures with freedom from pitting corrosion of steel heat transfer surfaces and from boiler scale due to Patented Dec. 5, 1967 hard water salts of calcium and magnesium by the use of boiler feed water which is alkalized with lithium hydroxide to a pH in the range of about 9 to 12, preferably in the range of 10 to 11, and contains an amount of ethylenediamine tetraacetic acid as the neutral lithium salt which is in small excess of that required for chelation with all of the polyvalent metal ion in the feed and boiler water, for example, an amount which is in excess of the chelating equivalent by from about 5 to 15 ppm.

Practice of the method of the invention does not require any departure from general procedures observed in operating high pressure steam generators. The lithium hydroxide and lithium salt of the ethylenediamine tetraacetic acid (EDTA) may be added as a single aqueous solution to the feed water. This aqueous solution may be prepared by adding the required amount of ethylenediamine tetraacetic acid to a solution in water of an amount of lithium hydroxide which is sufficient to both neutralize the EDTA and supply alkalinity to the feed water to the required pH value.

In operation of the method of the invention the ethylenediamine tetraacetic acid of the lithium salt enters into chelation with polyvalent metal ions of the feed and boiler water to form water-soluble metal chelates which remain in solution in the boiler water. A resultant of this chelation is the sequestering of calcium and magnesium ions whereby scale formation on the steel heat transfer surfaces from hard water salts is avoided.

A further advantage of the use of the lithium salt of ethylenediamine tetraacetic acid with lithium hydroxide in accordance with the method of the invention is the provision of more adequate protection against corrosion, including pitting corrosion by the lithium enriched spinel film, thickness for thickness of the film, than is provided by the lithium enriched spinel film formed in the method of the aforesaid patent. A lithium enriched spinel film is formed over the whole surface of the steel tubes in contrast to only localized, spot formation as in the method of the aforesaid patent. Further, formation of the lithium enriched spinel film occurs at the low con centrations of LiOH in the boiler water which correspond to an alkalinity in the pH range of 9 to 12.

The method of the invention may be practiced with benefit of the foregoing advantages in the operation of steam generators at high pressures which may range from a low of around 350 p.s.i. to a median of about 1400 to 1500 psi. and higher.

Since various changes and modification may be made in the practice of the invention without departing from the spirit or scope thereof, it is not intended to be limited except as may be required by the appended claims.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. In the operation of a steam generator at high pressures of at least 350 psi. in which the steam is generated by contacting the water with a heated steel surface, the improvement which comprises operating said generator at said high pressures with boiler feed water which is alkalized with lithium hydroxide to a pH in the range of 9 to 12 and contains in solution an amount of the neutral lithium salt of ethylenediamine tetraacetic acid which is in small excess of that required for chelation of all the polyvalent metal ions in the feed and boiler water.

2. In the operation of a steam generator at high pressures as defined in claim 1, wherein the amount of the neutral lithium salt of ethylenediamine tetraacetic acid 3 4 is in exces of the che!ation equivalent for all the poly- References Cited vaient metal ions by from about 5 to 15 ppm. UNITED STATES PATENTS 3. In the operation of a steam generator at high pressures as defined in claim 1, wherein the boiler feed water 3 173,404 3/1965 Bl 3 1 122 379 is aikalized with lithium hydroxide to a pH of about 5 10 to 11. KENNETH W. SPRAGUE, Primary Examiner. 

1. IN THE OPERATION OF A STEAM GENERATOR AT HIGH PRESSURES OF AT LEAST 350 P.S.I. IN WHICH THE STEAM IS GENERATED BY CONTACTING THE WATER WITH A HEATED STEEL SURFACE, THE IMPROVEMENT WHICH COMPRISES OPERATING SAID GENERATOR AT SAID HIGH PRESSURES WITH BOILER FEED WATER WHICH IS ALKALIZED WITH LITHIUM HYDROXIDE TO A PH IN THE RANGE OF 9 TO 12 AND CONTAINS IN SOLUTION AN AMOUNT OF THE NEUTRAL LITHIUM SALT OF ETHYLENDIAMINE TETRAACETIC ACID WHICH IS IN SMALL EXCESS OF THAT REQUIRED FOR CHELATION OF ALL THE POLYVALENT METAL IONS IN THE FEED AND BOILER WATER. 